a) Field of the Invention
The present invention relates to charged particle beam exposure, and more particularly to a charged particle beam exposure method using a blanking aperture array.
b) Description of the Related Art
ICs are now expected to be the kernel of technical advancement in all industrial fields. The integration density of ICs improves about four times in two or three years. For example, the integration of dynamic random access memories DRAMs has been improved from 1M, 4M, 16M, 64M, 256 M and to 1G.
Such high integration relies greatly upon fine pattern lithography. An optical technology has advanced to allow a fine patterning of 0.5 .mu.m line width. However, the optical technology has a limit in the order of about 0.3 .mu.m line width. A high precision has gradually become difficult in forming a contact hole and aligning it with an underlying pattern.
With electron beam exposure, a fine patterning of 0.1 .mu.m line width or thinner can be performed at an alignment precision of 0.005 .mu.m or better. With the block exposure or the blanking aperture array type exposure proposed by some of the present inventors and others, throughput of 1 cm.sup.2 /1 sec can be expected. In the block exposure, an electron beam is shaped in a desired pattern by a stencil (selective transmission) mask and applied to an exposure surface. In the blanking aperture array type exposure, an exposure surface is scanned on a row unit basis by an electron beam passed through a mask having a number of apertures each of which can control the electron beam to transmit or intercept.
The electron beam exposure has prominent advantages over other lithography techniques, when taking into consideration the prospective improvement in any one of fine patterning, alignment precision, turn around, reliability, and software.
The electron beam exposure generally one of a point beam, a variable rectangle shaped beam, a block pattern beam, or the like. Each electron beam is required to be controlled in accordance with a desired pattern to be exposed on a particular area of a wafer.
An electron beam exposure apparatus capable of performing a block exposure will be described with reference to FIG. 13.
The exposure apparatus is divided mainly into an exposure unit 10 and a control unit 50. The exposure unit generates an electron beam, shapes it in a spot or pattern, and exposes a desired area of a subject, The control unit 50 generates control signals for controlling the exposure unit 10. Under the exposure unit 10, a stage 35 is provided on which a subject W to be exposed is placed.
First, the exposure unit 10 will be described. Electrons generated from a cathode electrode 11 are pulled out by a grid electrode 12 and anode electrode 13. These electrodes 11, 12, and 13 constitute a charged particle beam source 14.
An electron beam generated by the charged particle beam source 14 is shaped by a first slit having, for example, a rectangular aperture. The electron beam passes a first focusing electron lens 16, and enters a slit deflector 17 which deflects the beam to adjust the beam impinging position on a stencil mask 20. The slit deflector 17 is controlled by a deflection adjusting signal S1.
In order to shape the electron beam in a desired pattern, the stencil mask 20 has a plurality of apertures, such as rectangle apertures and block apertures. The electron beam passed through the slit deflector 17 enters an electron beam shaping section whereat the beam is shaped in a desired pattern. The electron beam shaping section has second and third electron lenses 18 and 19 facing each other, the stencil mask 20 mounted between the electron lenses 18 and 19 and movably in the horizontal direction, and first to fourth deflectors 21 to 24 mounted above and under the stencil mask 20 for selecting one of the apertures of the stencil mask 20 by deflecting the electron beam in response to position data P1 to P4.
The shaped electron beam is intercepted or passed by a blanking electrode 25 to which a blanking signal SB is applied.
The electron beam passed by the blanking electrode 25 is controlled by a fourth electron lens 26, aperture 27, refocusing coil 28, and fifth electron lens 29, and enters a focus coil 80 which focuses the electron beam on the surface of the subject to be exposed. An astigmatism coil 31 corrects astigmatism.
The electron beam then enters a sixth electron lens 32 whereat the beam is controlled by a main deflector 33 and a sub-deflector 34 and applied to a desired area on the subject W to be exposed. The main deflector 33 is an electromagnetic deflector supplied with an exposure position determining signal S2, and the sub-deflector 34 is an electrostatic deflector supplied with an exposure position determining signal S3.
The subject W to be exposed is placed on the stage 35 movable in X and Y directions. The exposure unit 10 also has first to fourth alignment coils 36, 37, 38, and 39.
The control unit 50 has a memory 51 such as a magnetic tape and disk, and a central processing unit (CPU) 52. Data for designing an integrated circuit is stored in the memory 51. CPU 52 reads the data and processes it. CPU 52 also controls the whole system of the charged particle exposure apparatus.
An interface 53 transfers various exposure information from CPU 52. The exposure information includes exposure pattern information of a pattern to be exposed on a wafer W, mask information of the stencil mask 20, and other information. A data memory 54 stores the exposure pattern information and mask information transferred from the interface 53. A pattern controller 55 includes designating means, holding means, calculating means, and outputting means to perform various processes in accordance with the exposure pattern information and mask information read from the data memory 54. For example, one of such processes is a process of designating one of apertures of the stencil mask, generating position signals P1 to P4 representing the position of the designated aperture on the stencil mask, arid calculating a correction value H for correcting a difference between the pattern to be exposed and the designated aperture pattern. A DAC/AMP unit 56, having a digital-to-analog conversion function and a signal amplification function, generates a deflection adjusting signal S1 in accordance with the correction value H. A mask transport mechanism 57 moves the stencil mask 20, when necessary, in response to a signal from the pattern controller 55.
A blanking controller 58 controls a DAC/AMP unit 55 having a digital-to-analog conversion function and a signal amplification function to output the blanking signal SB from the DAC/AMP unit, in response to a signal from the pattern controller 55.
A sequence controller 60 controls the exposure sequence in accordance with the exposure pattern information sent from the interface 53. A stage transport mechanism moves the stage 35, when necessary, in response to a signal from the sequence controller 60.
The motion of the stage 35 is detected with a laser interferometer 62, and the motion information is supplied to a deflection controller 63. The deflection controller 63 calculates the exposure position on the wafer W, supplies signals to DAC/AMP units 64 and 65 DAC/AMP units 64 and 65 each have a digital-to-analog Conversion function and signal amplification function and generate the exposure position determining signals S2 and S3, respectively The deflection controller 63 also supplies a signal to the sequence controller 60.
In an ordinary electron beam exposure, the electromagnetic main deflector 33 deflects an electron beam within a main deflection field of 2 to 10 mm square, and the electrostatic sub-deflector 34 deflects an electron beam within a sub-deflection field of 100 .mu.m square. The main deflector is used to move a beam position from one main deflection field to another main deflection field, for example, intermittently, because of its low operation speed. The sub-deflector is used to move the beam position within each sub-deflection field.
Pattern data read by CPU 52 from the memory 51 is transferred to and stored in the data memory 54. The pattern controller 55 reads the pattern data in the data memory 54 and resolves it into sub-pattern data for each exposure shot.
In accordance with the resolved sub-pattern data for each exposure, data for the main deflector 33, data for the sub-deflector 34, data for the slit deflector 17, blanking signal SB, and the like are generated to deflect the electron beam.
As described previously, some of the present inventors and others have proposed the block pattern exposure method which controls the cross sectional shape of an electron beam by using the above-described stencil mask, and the blanking aperture array type exposure method which controls a number of apertures having the same shape independently for the pattern exposure. With the latter method, a desired pattern is exposed by assigning the whole area of a subject with minute areas and by turning on and off an electron beam to each fine area.
FIG. 14A is a schematic plan view showing an example of a blanking aperture array (BAA) with a number of apertures 81 being formed in a light shielding substrate 80.
For example, at the top row LA1 shown in FIG. 14A, sixty four apertures 81 are formed, and at the next row sixty four apertures 81 are formed in a checkerboard or staggered pattern relative to those at the top row LA1. Constituting one pair by two aperture rows LA1 and LB1, similar eight pairs of aperture rows are disposed in total in the column direction.
The pattern of the mask is reduced in size by 1/500 on the surface of a specimen. Each aperture 81 is a 25 .mu.m square. The apertures are disposed at a 50 .mu.m pitch in the row direction, and at a 100 .mu.m pitch in the column direction. In the column direction, one aperture row staggered from the upper and lower rows is interposed between this 100 .mu.m pitch.
The dimension of the blanking aperture array is about 3200 .mu.m in the row direction, and about 800 .mu.m in the column direction. An aperture of a 25 .mu.m square is reduced to a 0.005 .mu.m square on the subject surface.
The whole area of a subject can be exposed by a pair of aperture rows LA1 and LB1 by applying a charged particle beam through each aperture 81 while moving the blanking aperture array in the column direction. With the eight pairs of aperture rows, the whole area of the subject can undergo a multiple exposure of eight times.
Each aperture 81 is formed with electrodes 82a and 82b on opposite sides of the aperture. The electron beam passing through the aperture 81 can be deflected to an area outside of the subject by applying suitable voltages to the electrodes 82a and 82b.
In other words, the electrodes 82a and 82b function as a shutter for an electron beam passing through the aperture 81. A blanking aperture BA is constituted by an aperture 81 and two electrodes 82a and 82b formed on respective opposite sides of the aperture.
In the example described above, 128.times.8 blanking apertures BA are formed. Eight blanking apertures BA are disposed in the column direction at the same column position. Therefore, the same area on the subject can be exposed eight times.
FIG. 14B is a schematic diagram illustrating a method of exposing a broad exposure area by using the blanking aperture array BAA. The exposure area is divided into a plurality of stripe areas ST1, ST2, . . . Each stripe area, for example, ST1, is divided into a plurality of crossed sub-areas SR1, SR2, . . . Each sub-area SR is formed of a series of subfields. The sub-field has a dimension, 100 .mu.m.times.100 .mu.m, for example sufficient for the sub-deflector to scan this area as shown in righthand side of FIG. 14B.
Each sub-field is divided into a plurality of segment areas SG longer in one direction. For example, the sub-field is divided into twenty segment areas. The width of the segment area SG is set equal to or narrower than the width of the image of the blanking aperture array. In the abovedescribed example, the width is 6.4 .mu.m or narrower.
The stage, holding a subject to be exposed, is driven vertically as indicated by a large multi-folded arrow. An electron beam passed through BAA is controlled by the sub-deflector to scan each segment area SG in the column direction indicated by a thin straight solid arrow. After scanning the segment area SG1 from the bottom to top, the next segment area SG2 is scanned from the bottom to top. When all the segment areas of the sub-field are scanned, the main deflector is driven to move to the next righthand side sub-field.
The motion of the stage indicated by the large folded arrow is 25 mm/sec, for example, and the motion of an electron beam on the exposure surface is 0.5 .mu.m/50 nsec.
FIG. 14C illustrates how the exposure is performed by eight pairs of aperture rows of BAA. The abscissa represents time in nsec, and the ordinate represents a distance on the specimen surface in .mu.m.
It is assumed that a specimen moves at a constant speed of 0.5 .mu.m/50 nsec relative to the blanking aperture array. A stepwise line indicated by LA1 shows the exposure by one blanking aperture BA at the row LA1 shown in FIG. 14A.
The area on the specimen from the reference position to 0.05 .mu.m is exposed during the first 5 nsec, and the area from 0.05 .mu.m to 0.1 .mu.m is exposed during the next period from 5 nsec to 10 nsec.
The area under the blanking aperture is sequentially exposed every 5 nsec, and an exposed stripe is formed in the column direction on the specimen surface.
Assuming that the specimen moves from the top to bottom of BAA shown in FIG. 14A, the area on the specimen surface at one column appears after 10 nsec under another blanking aperture staggered relative to the preceding blanking aperture.
An area which the blanking aperture row LA1 exposes, for example, is lines and spaces having 0.5 .mu.m width and 0.1 .mu.m pitch.
In order to expose all the area, it is necessary to use both the blanking aperture row LA1 and the next blanking aperture row LB1 disposed complementarily or in a checkerboard pattern relative to the row LA1. A broken line indicated by LB1 in FIG. 14C shows the exposure by one blanking aperture BA at the row LB1 shown in FIG. 14A.
After 20 nsec, the area exposed by the first blanking aperture row LA1 at time 0 appears under the third blanking aperture row LA2. Similarly, after 40 nsec, 60 nsec, . . . , the same area appears under-the blanking apertures BA of the successive rows. By exposing at these timings, the same area can be exposed multiple times.
The exposure speed 0.5 cm.sup.2 /sec is obtained for the stage motion speed 25 mm/sec. For the exposure speed 1 cm.sup.2 /sec, the stage motion speed is set to 50 mm/sec.
With the multiple exposure, the same area is exposed multiple times, for example, eight times. As a result, current is turned on and off in a fraction of total change, and an electron beam as a whole gradually increases and decreases. Namely, a rapid change of an exposure current can be prevented, providing an easy refocus for the compensation of the Coulomb interaction.
FIGS. 15A to 15J are schematic diagrams explaining an exposure using a blanking aperture array. For the simplicity of description, a pattern is exposed only once. In the multiple exposure, similar exposures are repeated.
It is assumed, as shown in FIG. 15A, that two rows of blanking apertures BA1 to BAS are disposed in a staggered pattern.
The pattern to be exposed is as shown in FIG. 15B. This pattern shown in FIG. 15B is exposed on a subject. The subject is assumed to move from the top to bottom of the blanking aperture array shown in FIG. 15A.
First, as shown in FIG. 15C, the bottom areas of the subject to be exposed reach under i.e., are disposed beneath the row of blanking apertures BA1 and BA2. These apertures BA1 and BA2 are turned on to expose the areas.
At the next timing as shown in FIG. 15D, the areas to be exposed are disposed under the upper row of the three blanking apertures BA1, BA2, and BA3. These areas are then exposed.
As the second next timing as shown in FIG. 15E, the areas to be exposed are disposed under the upper row of the two blanking apertures BA1 and BA2. These areas are then exposed.
At this timing, there are other areas to be exposed, adjacent the areas under the blanking apertures BA1 and BA2. However, these other areas have no corresponding aperture at this timing, and so no exposure is performed for these other areas.
In the above manner, while the specimen moves downward under the blanking apertures BA, the areas under the blanking apertures are selectively exposed.
After exposing the three areas as shown in FIG. 15F, the two areas not exposed at the timing shown in FIG. 15E are disposed under the row of the two blanking apertures BA4 and BAS as shown in FIG. 15G. At this timing, these areas under the blanking apertures BA4 and BA5 are exposed.
On the five columns of the pattern, the first, third, and fifth-columns are exposed using the upper row blanking apertures BA1, BA2, and BA3, and the second and fourth columns are exposed using the lower row blanking apertures BA4 and BA5 at delayed timings.
As the subject moves thereafter as shown in FIGS. 15H, 15I, and 15J, the areas to be exposed are sequentially exposed to form the pattern shown in FIG. 15B.
Using the blanking aperture array, therefore, an arbitrary pattern can be exposed.
The blanking aperture array type exposure is not a substitute for the block exposure, and instead they are both used in a compatible manner. Namely, both the block pattern and blanking aperture array may be formed on a stencil mask to expose a frequently used pattern with the block exposure and a less used pattern with the blanking aperture array exposure.
As described above, with the blanking aperture array, an arbitrary pattern can be exposed by controlling each blanking aperture when applying a charged particle beam to a particular area on the specimen surface.
However, with the blanking aperture array (BAA) method, the amount of data becomes enormous if the abovedescribed specification is applied, because one hundred and twenty eight data are transferred every 5 nsec to 2.5 nsec, and on/off information (i.e., a bit map) of 2000 rows of 256 K bits is used every 10 .mu.sec or 5 .mu.sec.
Such bit information is practically unable to be stored in a bit map memory, because there are 4.times.10.sup.5 .times.4.times.10.sup.5 =160 G cells of 0.05 .mu.m square within a 20 mm square and it is difficult to access such a high capacity memory every 2.5 nsec.